System.NotSupportedException : Cannot create boxed ByRef-like values.

[tonyranieri]

Trying to test code that uses a service that returns ReadOnlySpan<char> doesn't seem possible.

Using Moq Version 4.10.1

I've created a repo that reproduces the issue - https://github.com/wessiyad/moq-readonlyspan-issue

PS C:\source\poc\moq-ros-issue\src> dotnet test
Skipping running test for project C:\source\poc\moq-ros-issue\src\WebSample\WebSample.csproj. To run tests with dotnet test add "<IsTestProject>true<IsTestProject>" property to project file.
Build started, please wait...
Build completed.

Test run for C:\source\poc\moq-ros-issue\src\Test\bin\Debug\netcoreapp2.2\Test.dll(.NETCoreApp,Version=v2.2)
Microsoft (R) Test Execution Command Line Tool Version 15.9.0
Copyright (c) Microsoft Corporation.  All rights reserved.

Starting test execution, please wait...
[xUnit.net 00:00:01.09]     Test.ValuesControllerTest.Fails_At_Runtime [FAIL]
Failed   Test.ValuesControllerTest.Fails_At_Runtime
Error Message:
 System.NotSupportedException : Cannot create boxed ByRef-like values.
Stack Trace:
   at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean wrapExceptions, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor)
   at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean wrapExceptions, Boolean skipCheckThis, Boolean fillCache)
   at System.Activator.CreateInstance(Type type, Boolean nonPublic, Boolean wrapExceptions)
   at Moq.LookupOrFallbackDefaultValueProvider.GetDefaultValue(Type type, Mock mock) in C:\projects\moq4\src\Moq\LookupOrFallbackDefaultValueProvider.cs:line 127
   at Moq.Mock.GetDefaultValue(MethodInfo method, DefaultValueProvider useAlternateProvider) in C:\projects\moq4\src\Moq\Mock.cs:line 909
   at Moq.Return.Handle(Invocation invocation, Mock mock) in C:\projects\moq4\src\Moq\Interception\InterceptionAspects.cs:line 343
   at Moq.Mock.Moq.IInterceptor.Intercept(Invocation invocation) in C:\projects\moq4\src\Moq\Interception\Mock.cs:line 20
   at Castle.DynamicProxy.AbstractInvocation.Proceed()
   at Castle.Proxies.IFooServiceProxy.GetSomeData()
   at WebSample.Controllers.ValuesController.Get() in C:\source\poc\moq-ros-issue\src\WebSample\Controllers\ValuesController.cs:line 25
   at Test.ValuesControllerTest.Fails_At_Runtime() in C:\source\poc\moq-ros-issue\src\Test\ValuesControllerTest.cs:line 17

Total tests: 1. Passed: 0. Failed: 1. Skipped: 0.
Test Run Failed.
Test execution time: 2.1398 Seconds

.NET Core SDK (reflecting any global.json):
 Version:   2.2.203
 Commit:    e5bab63eca

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.14393
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\2.2.203\

Host (useful for support):
  Version: 2.2.4
  Commit:  f95848e524

.NET Core SDKs installed:
  1.0.0 [C:\Program Files\dotnet\sdk]
  1.0.1 [C:\Program Files\dotnet\sdk]
  1.0.4 [C:\Program Files\dotnet\sdk]
  2.0.0 [C:\Program Files\dotnet\sdk]
  2.0.2 [C:\Program Files\dotnet\sdk]
  2.1.4 [C:\Program Files\dotnet\sdk]
  2.1.300 [C:\Program Files\dotnet\sdk]
  2.1.302 [C:\Program Files\dotnet\sdk]
  2.2.104 [C:\Program Files\dotnet\sdk]
  2.2.203 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 1.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.1.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

Expected outcome is that I'm able to mock the method. When I discovered this I was simply trying to verify if the method was called or not, but I could see more involved testing where the result is returned as well.

Let me know if you need more details! 😀

[stakx]

Hi @wessiyad, thanks for reporting. At first glance, it looks like this is a limitation of Activator.CreateInstance, meaning that there is probably nothing we can do inside Moq to get this use case working.

[stakx]

Indeed, your repro code can be shortened to the following one-liner:

_ = Activator.CreateInstance(typeof(ReadOnlySpan<char>));

And I'm sorry to say that there's nothing we can do to work around this runtime / framework limitation. Moq won't be able to create a default return value for your interface method due to that.

ReadOnlySpan<T> is a ref struct (i.e. a stack-only type) and therefore cannot be boxed (i.e. wrapped in an object that lives on the heap), which would have to happen because Activator.CreateInstance (like many other reflection facilities) returns an object.